Nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries have already been put into practical use as batteries for small-size electronic devices such as laptop computers and cell phones, or the like, thanks to such advantages as their high energy density, low self-discharge, excellent long-term reliability and the like. Further, in recent years, utilization of the nonaqueous electrolyte secondary batteries has expanded to storage batteries for electric cars, household storage batteries and power storage batteries.
The lithium ion secondary batteries are constituted of a positive electrode containing mainly a positive electrode active substance, a negative electrode having, as its main component, a material capable of occluding and releasing lithium ions, and a nonaqueous electrolyte solution. As the positive electrode active substance to be used for the positive electrode, a lithium metal oxide such as LiCoO2, LiMnO2, LiNiO2, LiFePO4 or LiMn2O4 is used, for example.
Further, as the negative electrode active substance to be used for the negative electrode, metallic lithium, or silicon, an oxide such as silicon oxide or a carbonaceous material capable of occluding and releasing lithium ions are used. Lithium ion secondary batteries particularly using graphite (artificial graphite, natural graphite) or a carbonaceous material such as coke capable of occluding and releasing lithium ions have already been put into practical use.
On the other hand, as the nonaqueous electrolyte solution, one in which a lithium salt such as LiPF6, LiBF4, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2C2F5)2 or lithium bis(oxalate)borate (LiB(C2O4)2) is added to a mixed solvent of a cyclic carbonate solvent such as ethylene carbonate or propylene carbonate and a chain carbonate solvent such as dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate is used, for example.
In secondary batteries using such a nonaqueous electrolyte solution, for example, on the electrode surface of the negative electrode, a solvent in the electrolyte solution causes a reductive decomposition, whereby the decomposition product deposits on the negative electrode surface to increase the resistance, and gases generated by the decomposition of the solvent bulge the battery. Further on the electrode surface of the positive electrode, the solvent causes an oxidative decomposition, whereby the decomposition product deposits on the positive electrode surface to increases the resistance, and gases generated by the decomposition of the solvent bulge the battery. Consequently, decreases in the storage characteristics of the batteries and decreases in the cycle characteristics of the secondary batteries occur to arise a problem of decreases in the battery characteristics.
In order to prevent occurrence of these problems, a compound having a function of forming a protective coating, for example, vinylene carbonate, fluoroethylene carbonate or maleic anhydride is added to the nonaqueous electrolyte solution. Specifically, it is known that the decomposition of the compound added in the electrolyte solution is intentionally promoted on an electrode active substance surface during the initial charge time, and its decomposition product forms a protective coating having a protection function to prevent another decomposition of the solvent, that is, forms an SEI (Solid Electrolyte Interface). Then, it is reported that formation of the protective coating suitably suppresses the chemical reaction and the decomposition of the solvent on an electrode surface and consequently to exhibit an effect of maintaining battery characteristics of a secondary battery (Non Patent Literature 1). These additives are generally considered, however, to form an SEI on the negative electrode surface, and are insufficient to suppress gas generation and the like due to the oxidative decomposition of the solvent on the positive electrode.
Further, recently, in order to realize secondary batteries having a high energy density, use of positive electrodes having a high potential has been studied. For example, it is stated in Patent Literature 1 (JP2013-254605A) that a lithium metal composite oxide having a layered rock salt structure represented by Li1.19Mn0.52Fe0.22O1.98 is used for a positive electrode active substance; and in Patent Literature 2 (WO2012/141301) that a lithium metal composite oxide represented by LiNi0.5Mn1.5O4 is used for a positive electrode active substance. Since such lithium ion secondary batteries using positive electrodes having a high potential have higher voltages (4.5 V or more) than voltages of conventional common lithium secondary batteries (3.5 to 4.2 V), gas generation due to the oxidative decomposition of solvents on their positive electrodes is liable to occur.
On the other hand, as a method for suppressing the oxidative decomposition of a solvent in an electrolyte solution, an attempt is made in which the electrolyte solution is made to contain a fluorine-containing ether (Patent Literature 3: JP2004-363031A).